1. Field of the Invention
The present invention relates to an electrostatic latent-image developing toner.
2. Description of the Related Art
The electrostatic latent-image developing toner, which is used in an image-forming apparatus of an electrophotographic system, contains at least a resin and a colorant, and normally, a wax is added thereto in order to prevent high-temperature offset as well as low-temperature offset. The high-temperature offset refers to a phenomenon in which inter-toner aggregating force of fused toner to form a toner image is weakened due to overheat and one portion of the toner image is transferred onto a fixing roller, with the result that the toner adheres to the next recording medium (paper or the like). The low-temperature offset refers to a phenomenon in which heat energy to be used for fusing toner becomes insufficient to cause unfused toner in the vicinity of a recording medium with only the toner in the vicinity of the fixing roller being fused so that the adhesivity between the fixing roller and the toner becomes greater than the adhesivity between the toner and the recording medium to cause the toner image to adhere to the fixing roller, with the result that the subsequent toner adheres to the next recording medium. When the colorant and the wax are exposed to the surface of toner particles, the colorant and the wax tend to be transferred onto members such as a photosensitive member and a developing roller that contact the toner particles to cause image noise such as filming, which forms a main problem with the toner.
With respect to the toner, there have been demands for a low-temperature fixing property and a heat-resistant storing property, which are contradictory properties. In other words, the low-temperature fixing property is a property that allows a toner image to be sufficiently fixed on a recording medium at a comparatively low temperature, and in order to improve such a low-temperature fixing property, a method for using a resin having a comparatively low melting temperature as the toner constituent resin is proposed. However, the application of such a method results in a decrease in the glass transition point of the resin and the subsequent degradation in the heat-resistant storing property to cause aggregation at the time of storage at comparatively high temperatures. This also causes degradation in the image storing property. The image storing property is related to the storing property of the recording medium bearing an image that has been subjected to electrophotographic processes. In the case when the image storing property deteriorates, upon storing superposed images (in particular, double sided copies) at a high temperature (for example, 50° C.), the recording media adhere to each other, causing image separation when detached from each other.
With respect to toner particles having a core-shell shell structure in which a shell layer is formed on the surface of core particles so as to achieve both of the low-temperature fixing property and the heat-resistant storing property, a technique has been proposed (see Japanes Patent Application Laid-Open No. 2002-229251 and Japanese Patent Application Laid-Open No. 2002-229248) in which a low-softening-point resin is used as a core material, while a high-softening-point resin is used as a shell material. This method tries to improve the low-temperature fixing property by lowering the softening point of the core to decrease the viscosity, while improving the heat-resistant storing property by using a high-softening-point substance as the shell. With respect to the high-softening-point resin, generally known amorphous resins may be used. With this method, when the colorant and the wax are added to the core particles, the colorant and the wax are not exposed to the surface of toner particles because of the existence of the shell layer, thereby making it possible to effectively prevent image noise such as filming. In this method, however, the existence of the shell layer makes the core difficult to dissolve, failing to provide a sufficient low-temperature fixing property. Since the time required for the wax to elute to the surface of toner particles is too long, it is not possible to sufficiently prevent high-temperature offset. The resulting problem is that the fixing temperature range (non-offset temperature width) in which neither low-temperature offset nor high-temperature offset takes place is narrowed. The toner particles having the core-shell structure are generally obtained by adhering/fusing shell-use resin particles onto the surface of core particles in an aqueous medium; however, since the surface of the toner particles is formed by the high-softening-point shell-use resin particles, it becomes difficult to increase the fusing property of the toner particles and the degree of roundness in the entire toner particles.
In order to improve the low-temperature fixing property of the toner particles of the core-shell structure, a technique has been proposed (Japanese Patent Application Laid-Open No. 2002-341586) in which 2 to 15% by weight of crystalline resin is added to the shell layer. However, from the viewpoint of balance with the heat-resistant storing property, this structure has failed to exert a sufficient low-temperature fixing property.
It has been known that, upon transferring a toner image from an intermediate transferring member or a photosensitive member onto a recording medium to be fixed thereon, a simultaneous transferring/fixing system, which simultaneously carries out a transferring process and a fixing process, is preferably used from the viewpoints of a small size of the device and an improvement of the transferring efficiency (see Japanese Patent Application Laid-Open No. 2002-341584). However, the conventional toner particles fail to exert a sufficient low-temperature fixing property from the viewpoint of ensuring proper balance between the low-temperature fixing property and the heat-resistant storing property, and consequently fail to effectively reduce the fixing temperature even when applied to the above-mentioned simultaneous transferring/fixing system. For this reason, repeated image-forming processes cause a roughened surface of the intermediate transferring member, resulting in a problem of gloss irregularities.
In the case of a mono-component developing system in which, upon developing a latent image on the photosensitive member, the toner particles are allowed to pass through a gap between a developing member and a regulating member, such as a sleeve and a roller, to be friction-charged, the toner particles need to have an anti-braking property against mechanical stress. However, the above-mentioned toner particles having a core-shell structure are susceptible to separation of the shell layer, and have a problem with anti-breaking property. In the case when the anti-breaking property of the toner particles is poor, the toner particles are broken during developing processes to cause a widened charge-quantity distribution and the subsequent pollution inside the actual machine, as well as toner adhesion to the regulating blade and the subsequent longitudinal scratch lines.